It is known that a significant amount of air drag results when a vehicle travels over a roadway. This is due to an area of low pressure created at the rear end of the vehicle. For example, with a tractor-trailer type vehicle, the air makes a sharp bend around the squared-off back of the trailer, thus causing turbulence and drag. To overcome this problem, engine power is required and therefore fuel. Furthermore, the turbulence also causes poor visibility in rainy conditions and, an accumulation of dirt on the back of trailers, and a tangible buffeting of vehicles that are following the tractor-trailer.
In the trucking industry, the aerodynamics of vehicles is a growing concern. Tests indicate that fuel savings from aerodynamic improvements are equivalent to fuel savings attained by a chassis weight reduction and require fewer services to maintain. The air resistance or aerodynamic drag of the vehicle increases the power needed by the engine as the speed increases. It is known that a tractor-trailer needs about 100 HP to overtake the air drag at a speed of 55 MPH. Approximately half the energy reaching the drive wheels is required to overcome air resistance at cruising speed. Tests reveal that the square back end contributes approximately to 30% of the air drag. Therefore, the addition of an aerodynamic device, such as a rear deflector, diminishes the air drag and results in lower fuel consumption.
This disclosure pertains to the increasing need for reduction in operating costs and conservation of fuel in the commercial trucking industry. Blunt body parasitic drag has long been acknowledged as having a large impact on the aerodynamically efficiency of commercial straight trucks and trailers. Some work from the published literature is presented first followed by a review of pertinent patents.
Sovran et al in Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles; Plenum Press, NY; 1978 reported that Mason and Beebe in Mason, Jr., W. T., and P. S. Beebe. “The Drag Related Flow Field Characteristics of Trucks and Buses.” Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles General Motors Research Laboratories, 1978. pp. 45-93 examined several add-on devices aimed at increasing the base pressure of a tractor-trailer in 1976. They included vertical and horizontal splitter panels, guide vanes, and non-ventilated cavities. The vertical splitter panels were found to have little or no effect on the tractor-trailer drag. The turning vanes had an adverse effect on drag. Mason and Beebe surmised that the added drag of the turning vanes outweighed any gain from increased base pressure. The only device that showed positive results was the non-ventilated cavity design. Mason and Beebe varied the depth of the panels and found an optimum panel length of 0.55 m (21.8 inches) for a conventional 14.6 m (48 foot) trailer yielding an overall drag reduction of 5 percent.
Similar research by Hucho, W.; Aerodynamics of Road Vehicles; Butterworths, London; 1987 used a small minivan style vehicle in the 1970s to substantiate the drag reduction behavior of non-ventilated cavities. In 1985, Cooper reported in The Effect of Front Edge Rounding and Rear-Edge Shaping on the Aerodynamic Drag of Bluff Vehicles in Ground Proximity, SAE 850288 the results of extensive tests on detailed scaled models of a tractor trailer and a panel truck using rounded and beveled panels on the rear end of the vehicle. His results indicated an optimum reduction in the drag of 7-10% of the baseline model drag and indicated it was not unreasonable to expect this at full scale.
Many ideas regarding the passive reduction of drag on trailers have been patented and a review of the concepts most closely associated to the present idea is presented below in order to differentiate the novelty of the present idea from these previous concepts.
U.S. Pat. No. 4,006,932 in the name of McDonald (1977) claims an inflatable drag reducer for land vehicles. The device when inflated, however, does not form a large sealed aft cavity. Small cusp portions on all trailing edges form a large wall at the rear of the device, thus filling in the cavity. No provisions for removal of the device to allow entry to the rear of the trailer is mentioned or described.
U.S. Pat. No. 4,236,745 (Davis, 1980) utilizes the concept of a retractable or collapsible device of the fully enclosed surface type. The present proposed device is also collapsible, but not of the fully enclosed variety. Lechner, U.S. Pat. No. 5,375,903 (1994), also puts forth a self inflatable concept with stiffening supports. However this is also of the enclosed variety.
U.S. Pat. No. 4,142,755 by Keedy (1979) describes a class of aft devices that reduce the width and height of the aft end of the trailer with panels or surfaces to try to keep the flow attached to the proposed surface reducing the wake width and associated drag. The device of this disclosure is of the class of open-ended aft cavities.
Mulholland, in U.S. Pat. No. 4,458,936 (1984), employs rigid panels attached with hinged mechanisms to the rear of the trailer. These outer surfaces join together to form an apex behind the vehicle. The concept disclosed herein uses panels that can hinge if desired, but do not join together to form an enclosed exterior surface.
U.S. Pat. No. 4,682,808 was filed by Bilanin in 1985 on a variation of the cavity design concept that inset the end panels from the trailer perimeter. His claims indicated a drag reduction on the order of 10% is claimed. The Bilanin device includes three or four panels oriented in a box formation. Three of the panels (top and sides) are generally inset from the perimeter of the trailer base and that the bottom panel has no inset and is in line with the bottom of the trailer. All the panels are mounted perpendicular to the trailer base, however the Bilian patent does allow for inset angled side panels. This orientation of panels produces vortex structures in the area between the panel and the perimeter of the trailer in the manner of the intent of the Mason and Beebe splitter devices.
The optimum dimensions reported in the Bilanin publication were:                D/W=0.13, G/H4.1.15, D/L=G/L=0.3where D=plate inset from each of the sides of the trailerG=plate inset from the top of the trailerW=width of trailer,H=height of trailer,L=plate length        
With these criteria, Bilanin claims that a 10.2% reduction in drag was achieved on a typical tractor-trailer with a base tractor trailer CD of 0.6, however no test results have been published in the literature. Bilanin includes optimum ranges for the geometric ratios previously specified, in order to adjust for maximum efficiency of vortex capture and for structural members already existing on the trailer base. These ratio ranges are:
0.1 D/W5—0.2, 0.25 D/L=G/L0.4
Bilanin also specifies that the length of the plates should be between 40″ and 56″. The disclosure described herein can use angled panels in the dimensional ranges specified by Bilanin, but the panels are not inset from the edge, as per Bilanin, and in particular, the resulting cavity for the present design is not one but two separately sealed cavities that enable a viable device for real world applications.
U.S. Pat. No. 6,485,087 in the name of Roberge (2002) depicts a three sided non-ventilated cavity formed from angled fins off the rear of the trailer, aligned with the edge of the trailer. The present concept also utilizes angled fins set to the trailer edge, however, a completely enclosed cavity is formed once the device is deployed, which is further subdivided using eight fins to form two separate and sealed cavities. It should be particularly noted that the method for storage of the device by Roberge places it between the opened doors and the sides of the trailer during loading and unloading. This requires a trailer with doors configured to provide clearance for said device. The present device strategy does not have this constraint. Lastly, no method of sealing the adjoining panels or of sealing the panels to the rear of the trailer is mentioned by Roberge. The present concept requires a sealed cavity to be effective.
U.S. Pat. No. 6,257,654 in the name of Bolvin (2001) describes a similar concept to U.S. Pat. No. 6,485,087, but does not form a sealed or non-ventilated cavity.
U.S. Pat. No. 6,595,578 in the name of Calsoyds et al. describes a truck after-body reduction device having two cavities with rounded bottoms facing rearward. The patent does not discuss sealing, lengths or angles or any specific geometry or any kind of folding or attachment mechanisms
This disclosed invention uses a different approach to drag reduction and attachment methods, which, to the best of the authors' knowledge, has not previously been recognized. The disclosed device requires the rear cavity(ies) to be completely sealed and this is crucial to the functionality of the device. The current device also employs a unique method of stowage which can be used with rigid sided or collapsible sided devices and requires two separately sealed cavities.